Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which the brightness of a pixel is based on the intensity of the echo return. In a so-called xe2x80x9ccolor flowxe2x80x9d mode, the flow of blood or movement of tissue can be imaged. Conventional ultrasound flow imaging methods typically use either the Doppler principle or a time-domain cross-correlation method to estimate the average flow velocity, which is then displayed in color overlaid on a B-mode image.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The frequency shift of backscattered ultrasound waves may be used to measure the velocity of the back-scatterers from tissue or blood. The frequency of the backscattered signal increases when blood flows toward the transducer and decreases when blood flows away from the transducer, the amount of increase or decrease being proportional to the velocity of the blood flow. Thus this frequency shift may be used to estimate the average flow velocity, which is displayed using different colors to represent speed and direction of flow. The color flow velocity mode displays hundreds of adjacent sample volumes simultaneously, all color-coded to represent each sample volume""s velocity.
The resulting flow image may be combined with a stationary tissue (i.e., B-mode) image acquired by detecting either the fundamental or (sub)harmonic signal components, either by summation or as an overlay in order to provide anatomical landmarks. An advantage of the overlay is that it may be done in color so that the flow regions stand out clearly. However, this method requires a more complex display. Furthermore, the flash artifacts resulting from surrounding tissue motion are severe. Injection of the background B-mode image by summation (either coherent or incoherent) results in more benign flash artifacts. In either case, however, additional firings beyond those used for flow imaging are required to acquire the imaging data representing stationary tissue.
An alternative method for imaging blood flow is B-flow. In B-flow, high spatial resolution is achieved by using broadband pulses, while high frame rate is achieved by using small packet sizes. High SNR/dynamic range is maintained by using coded excitation. Flow sensitivity in the range direction is highest and arises from pulse-to-pulse RF decorrelation, while flow sensitivity in the cross-range direction is due to pulse-to-pulse amplitude decorrelation as a group of reflectors (e.g. blood or contrast agents) flows across the beam profile.
The method includes transmitting a small packet of coded broadband pulses with a given pulse repetition interval to a transmit focal position. The packet size is made small (e.g., 2-4 firings) to achieve high frame rate, with the undesirable side effect of reduced SNR. SNR can be optionally recovered using coded excitation. A coded sequence of broadband pulses (centered at a fundamental frequency) is transmitted multiple times to a particular transmit focal position, each coded sequence constituting one firing. On receive, the receive pulses acquired for each firing are decoded and bandpass filtered, e.g., to isolate a spatially compressed pulse centered at the fundamental frequency. The backscattered signals from this sequence of firings are then filtered in slow time to remove echoes from stationary or slowly moving reflectors along the transmit path. The slow-time filtering is preferably performed by a high-pass FIR (finite impulse response) or IIR (infinite impulse response) wall filter, which increases the flow signal-to-clutter ratio. A flow image is formed by scanning the transmit focal position across the region of interest. Frame rate may be increased by simultaneously processing more than one receive vector from a single transmit vector with parallel receive hardware. The packet size, pulse repetition interval (PRI) and region of interest (ROI) may be optionally controlled by the user.
When imaging blood flow of very small blood vessels often down to the perfusion bed of the organ, the signals are of such low amplitude and the blood moving so slowly that conventional flow imaging techniques are not able to extract an adequate flow signal. In cases like this, contrast agents such as gas-filled microbubbles are optionally injected into the blood to serve as markers for imaging blood flow. As previously described above, a coded sequence of broadband pulses is transmitted multiple times to a particular transmit focal position. Fundamental and (sub/ultra) harmonic signals are generated from interaction between the transmitted ultrasound pulses and the propagation medium, especially the injected contrast agents. On receive, the receive signals are decoded and bandpass filtered to selectively isolate the fundamental or (sub/ultra) harmonic signals. These isolated selectively filtered signals are then high-pass filtered across firings using a conventional wall filter. As a result of this filtering, selectively filtered signals reflected from non-stationary tissue or flow regions along the transmit path are extracted, while received energy at the frequencies which would have contributed to undesirable stationary tissue signal is suppressed.
The imaging modes described above reduce the signal from normal surrounding tissue to allow the user to effectively image signals substantially lower in amplitude. In the case of Coded Harmonic Angio (CHA) and Coded Angio (CA) modes, both developed for contrast agent imaging, the signal is that from injected contrast agents in very small blood vessels often down to the perfusion bed of an organ. In the case of B-flow mode, the user is able to image blood flow using (unlike Doppler) the high frequency, wideband imaging pulses commonly used for B-mode imaging, with frame rates that are comparable to B-mode.
Unfortunately, the absence of the surrounding tissue background that enables good visualization of these low lying signals makes it very difficult for the user to do active imaging, since the visualization of the anatomical landmarks allowing the user to optimally position the probe is limited. For certain applications, such as the early detection of certain vascular diseases, the limited tissue background imaging associated with the blood flow imaging is not ideal. More specifically, if the resolution of the background imaging was clear, the background image would provide a reference image that can be used by a sonographer to establish a reference for the anatomy being scanned. Unfortunately, using the present non-Doppler based methods of blood flow imaging described above often results in little or no background tissue imaging. Therefore, it would be desirable to provide the user a background tissue image, without sacrificing the system frame rate or destroying the imaging resolution of the blood flow low amplitude signals.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a method for displaying flow and reference background ultrasound images. The method comprises: transmitting at least first and second broadband pulses to a common transmit focal position; receiving at least first and second ultrasound reflections associated with the at least first and second broadband pulses; forming a flow and/or contrast agent signal component based on the at least first and second ultrasound reflections; forming a B-mode background signal component based on independent processing of at least one of the at least first and second ultrasound reflections; and displaying an ultrasound image including a flow/contrast agent image component in a first image portion of a display and a B-mode reference image component based on at least one of the at least first and second ultrasound reflections, said B-mode reference image component displayed in a second image portion of said display.
In an alternative embodiment, an ultrasound medical diagnostic system for imaging stationary and moving reflectors for an area of interest in a patient is disclosed. The ultrasound diagnostic system includes a transmitter for transmitting a sequence of at least two pulses to a transmit focal position and a receiver for receiving at least two echo signals associated with the sequence of at least two pulses. The echo signals contain a fundamental frequency component. The system further includes a first display processor configured to receive and process the at least two echo signals and is configured having a filter for supplying a filtered signal containing flow/contrast agent image information for moving reflectors based on the at least two echo signals. Also included is a second display processor configured to receive and independently process at least one of the at least two echo signals containing B-mode information for stationary reflectors based on the at least two echo signals, the B-mode information for stationary reflectors including said fundamental frequency component. A display is included for displaying a flow/contrast agent image of moving reflectors based on the filtered signal and a B-mode reference image of stationary reflectors based on the at least one of at least two echo signals supplied by the second display processor.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.